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Does harvest of the European grayling, Thymallus thymallus (Actinopterygii: Salmoniformes: Salmonidae), change over time with different intensity of fish stocking and fishing effort?

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Background. The European grayling, Thymallus thymallus (Linnaeus, 1758), is a fish species of high value in recreational fishing. The monitoring of changes in grayling populations is a high priority in fisheries. Data on the harvest of recreational anglers can potentially serve as an easy and inexpensive way to monitor changes in fish populations. This study aimed to assess spatio-temporal trends in catches of grayling in a larger geographical area. Materials and methods. This study analysed harvest rates of grayling by recreational anglers on 241 fishing grounds, in the Czech Republic, within 1986–2015 (30 years). Data from individual angling logbooks were used. The data were collected by individual anglers and processed by the Czech Fishing Union (Český rybářský svaz). Results. Over the period of 30 years, Czech anglers harvested a total of 9 928 grayling specimens weighing altogether 3 357 kg. Within the period surveyed, both parameters (the grayling biomass harvested and the representation of grayling in overall fish harvest) decreased to 10% of the initial values. The percentage of fishing grounds with a harvest of grayling decreased to 30% of the initial values. Harvest per effort decreased to 20% of the initial values over 11 years. There was only a weak correlation between fish stocking and fish harvest. There was a negative relation between the number of angler fishing visits with both catch (fish number) and yield (biomass) of grayling. The harvest was positively correlated with fishing effort. The mean size of harvested grayling remained constant (~0.35 kg) over 30 years. Conclusion. Harvest of grayling significantly declined over the last three decades, implying that increased effort in conservation of grayling is necessary. Future studies should focus on monitoring of the remaining self-reproducing grayling populations.
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DOES HARVEST OF THE EUROPEAN GRAYLING, THYMALLUS THYMALLUS
(ACTINOPTERYGII: SALMONIFORMES: SALMONIDAE), CHANGE OVER TIME
WITH DIFFERENT INTENSITY OF FISH STOCKING AND FISHING EFFORT?
Roman LYACH and Jiri REMR
Institute for evaluations and social analyses, Prague, the Czech Republic
Lyach R., Remr J. 2020. Does harvest of the European grayling, Thymallus thymallus (Actinopterygii:
Salmoniformes: Salmonidae), change over time with different intensity of sh stocking and shing
effort? Acta Ichthyol. Piscat. 50 (1): 53–62.
Background. The European grayling, Thymallus thymallus (Linnaeus, 1758), is a sh species of high value in
recreational shing. The monitoring of changes in grayling populations is a high priority in sheries. Data on the
harvest of recreational anglers can potentially serve as an easy and inexpensive way to monitor changes in sh
populations. This study aimed to assess spatio-temporal trends in catches of grayling in a larger geographical area.
Materials and methods. This study analysed harvest rates of grayling by recreational anglers on 241 shing
grounds, in the Czech Republic, within 1986–2015 (30 years). Data from individual angling logbooks were used.
The data were collected by individual anglers and processed by the Czech Fishing Union (Český rybářský svaz).
Results. Over the period of 30 years, Czech anglers harvested a total of 9 928 grayling specimens weighing
altogether 3 357 kg. Within the period surveyed, both parameters (the grayling biomass harvested and the
representation of grayling in overall sh harvest) decreased to 10% of the initial values. The percentage of shing
grounds with a harvest of grayling decreased to 30% of the initial values. Harvest per effort decreased to 20%
of the initial values over 11 years. There was only a weak correlation between sh stocking and sh harvest.
There was a negative relation between the number of angler shing visits with both catch (sh number) and yield
(biomass) of grayling. The harvest was positively correlated with shing effort. The mean size of harvested
grayling remained constant (~0.35 kg) over 30 years.
Conclusion. Harvest of grayling signicantly declined over the last three decades, implying that increased
effort in conservation of grayling is necessary. Future studies should focus on monitoring of the remaining self-
reproducing grayling populations.
Keywords: angling diaries, catch per unit effort, sheries management, population dynamics, salmonids
ACTA ICHTHYOLOGICA ET PISCATORIA (2020) 50 (1): 53–62
* Correspondence: Dr Roman Lyach, Institute for Evaluations and Social Analyses/Institut evaluací a sociálních analýz, 186 00 Prague/Praha, the Czech Republic, phone:
+ 420 737 242 256, e-mail: (RL) lyachroman2@gmail.com, lyachr@seznam.cz, (JR) jiri.remr@inesan.eu, ORCID: (RL) 0000-0001-7783-8256.
DOI: 10.3750/AIEP/02643
INTRODUCTION
In central Europe, the European grayling, Thymallus
thymallus (Linnaeus, 1758), is a sh species of high
value in recreational shing, commercial shing, and
species conservation. Grayling is a native sh species
that used to be common in streams and smaller rivers
located under mountain ranges (Persat 1996). However,
anglers, sheries managers, and environmentalists claim
that populations of grayling have been steadily decreasing
in central Europe over the last 20–30 years (Gum et al.
2009, Weiss et al. 2013, Mueller et al. 2018). Recently,
grayling has become one of the most threatened inland
freshwater sh species in central Europe. By studying
the effect of both natural and human-induced effects on
grayling populations, other authors discovered that the
main reasons for the decrease in grayling populations
are droughts, climate change, predation from sh-eating
birds and mammals, shing pressure, river damming and
straightening, loss of spawning substrates, land-use, and
pollution (Northcote 1995, Persat 1996, Uiblein et al.
2001, Gum et al. 2003, Duftner et al. 2005, Sternecker et
al. 2014, Geist and Hawkins 2016, Bierschenk et al. 2019).
By studying human–sh interactions, previous studies
have also found that the interaction between recreational
sheries and grayling is one of the most important drivers
of grayling populations (Duftner et al. 2005, Näslund et
al. 2005, 2010).
Data from angling logbooks can be used as proxy
data for changes in abundances of sh populations
(Sztramko et al. 1991, , Gudbergsson 2004, Jayasynghe
et al. 2006, Mosindy and Duffy 2007, Skov et al. 2017,
Kerr unpublished*). The results of the studies listed above
suggest that a change in harvest rate potentially suggests
a change in abundance in the ecosystem. In addition, data
from angling logbooks were previously used by other
authors to monitor sh abundances and populations,
Lyach and Remr
54
water quality, sh sizes, effects of water damming on sh
populations, and changes in water temperatures (Cowx
and Broughton 1986, Binet 1997, Lorenzen et al. 1998,
Draštík et al. 2004, Jayasynghe et al. 2006, Younk and
Perreira 2007, Zeeberg et al. 2008, Gerdeaux and Janjua
2009). The interaction between anglers and grayling is
crucial in the conservation of grayling. However, there
are only a few studies that describe the angler–grayling
interaction (Näslund et al. 2005, 2010). In addition, those
studies describe catches of grayling on small spatio-
temporal scale. No study describes the angler–grayling
interaction on a larger spatio-temporal scale in a larger
geographical area.
This study aimed to describe changes in the catch
(sh number) and yield (biomass) of grayling on a large
spatio-temporal scale (241 studied shing grounds, 30
years of data) in the Czech Republic. We expected that
the harvest of grayling was decreasing, mostly because
anglers, sheries managers, and environmentalists claim
that grayling populations have been declining. It was
hypothesized that both catch and yield would follow
and mirror this trend. Another aim of this study was to
describe changes in the number of shing grounds with
actual catches of grayling. We expected that smaller
grayling populations would perish over time, and that
human-induced effects would lead to a decreased number
of streams where self-sustainable grayling populations
exist. Another aim was to assess changes in the size of
caught grayling over time. We expected that anglers would
be catching smaller sh every year, mostly because the
angling and predation pressure on grayling populations
seems to be increasing. The last aim was to assess the
effect of sh stocking on the sh harvest. We expected
that shing grounds with higher stocking rates would also
display higher harvest rates.
MATERIALS AND METHODS
Study area. This study was carried out in the regions of
Prague (50°N, 014.5°E) and the Central Bohemian Region
(Středočeský kraj) (49.5°–50.5°N, 013.5°–015.5°E),
Czech Republic, central Europe (for map of the study area
see Lyach and Čech 2018a). Both regions together cover
an area of 11 500 km2. The region of Prague has mostly
urban character while the region of Central Bohemia has
a mostly rural character. The study area is dominated by
the rivers Elbe and Vltava. Both rivers belong to the upper
Elbe River basin. All rivers in the study area belong to
the North Sea drainage area. Studied shing grounds are
situated in lowlands of an altitude of 200–600 m above sea
level. Waters in the study areas are mostly mesotrophic
and eutrophic with biomass of 150–300 kg of sh per ha
(Lyach and Čech 2017b, 2018a, 2018b). The study area
includes salmonid streams (smaller streams, mostly <
10 m wide, usually dominated by salmonids) and non-
salmonid rivers (wider streams and rivers, usually 10–300
m wide, dominated by cyprinids or percids). Studied
rivers and streams are mostly at their carrying capacity
due to natural sh reproduction and intensive sh stocking
(Závorka et al. 2013). Grayling is a native sh species in
the study area.
Recreational shing in the Czech Republic. Recreational
shing in the Czech Republic is organized by the Czech
Fishing Union (Český rybářský svaz). The Union is the
principal authority in recreational shing in the Czech
Republic and is centralized for the whole country. Each
angler has to carry an angling logbook with him/her at all
times during shing. When an angler catches and keeps a
sh, he/she is obliged to write down the catch (identied
sh species, size of the sh in TL in cm, date of the catch,
and ID/name of the shing site). Filled logbooks are then
collected in January of the following year by the Czech
Fishing Union. Only anglers who submit their old lled
angling logbook will receive a new angling logbook for
the next year. Errors in lling of angling logbooks may
results into conscation of shing equipment, harvested
sh, or shing permit. Proper usage of angling logbooks
is checked in the eld by angling guards. Only killed
(harvested) sh are recorded in individual angling
logbooks. Fish that are undersized, caught during the
closed season, or otherwise released are not recorded in
logbooks. For a detailed description of recreational shing
in the Czech Republic see Lyach and Čech (2018a, 2018b).
Angling rules for grayling. Grayling, Thymallus
thymallus, is an important sh species in recreational
shing in the Czech Republic. The bag limit for salmonids
is either two sh or 7 kg of sh per angler per day,
whichever comes rst. Within 1986–2015 the minimum
legal angling size for grayling was 30 cm (TL, total
length). Any grayling that does not reach this size has to
be returned back to the water without any unnecessary
delay. All harvested graylings must be noted in individual
angling logbooks, including the date of catch, the weight
of sh, and the ID of the shing ground.
Grayling stocking. Annual stocking of grayling is
common and traditional in the study area. Most stocking is
performed on smaller salmonid streams and rivers (<10–
20 m wide) outside the main rivers. Grayling is mostly
stocked as 1–2 year sh (5–10 cm TL). Fish are usually
stocked in hundreds or thousands per stream. The main
goal of the sh stocking is to support wild populations.
Before sh stocking occurs, all stocked sh are weighed
together (in one bag) to the nearest 100 g. The number
of stocked sh is then estimated from the overall weight
by applying length–weight equations of the specic sh
species. The length–weight equations are based on data
from catches of a larger amount (at least 100) of sh that
were caught in the study area by sheries managers. Fish
stocking is performed by local sheries managers.
Data sources. Data from annual summaries of all
collected angling logbooks were used for this study. This
data originated from angling logbooks that were collected
from individual anglers. Fishing grounds are dened as
stream and river stretches where recreational shing can be
legally conducted. The selected shing grounds covered an
* Kerr S.J. 1996. A summary of Muskies Canada Inc. angler log information, 1979–1994. Technical Report TR-011. Ontario Ministry of Natural Resources, Kemptville
ON, Canada.
Harvest of grayling versus sheries management 55
area of 125 km2. This data was originally collected by the
Czech Fishing Union and later processed by the authors
of this study. Data from 241 shing grounds collected
within 2005–2015 (11 years) were used to analyse catch
and yield per shing effort (data on shing effort were
available only from the year 2005 onwards). For that
reason, the harvest of grayling over the years 1986–2004
was not related to shing effort in the analyses. Data from
years 2016 and 2017 were not used because the legislative
rules in recreational shing signicantly changed since
2016 (minimum legal angling size of grayling was
increased from 30 cm to 40 cm TL, total length). In the
rest of the analyses, data from 241 shing grounds within
1986–2015 (30 years) were used. A similar dataset was
previously used for scientic purposes (Humpl et al. 2009,
Jankovský et al. 2011, Boukal et al. 2012, Lyach and Čech
2017a, 2018a, 2018b).
Biometric data. This study assessed the overall catch
(number of sh individuals killed) and yield (total weight/
biomass of all sh killed), catch and yield per one hectare
of shing grounds, catch and yield per effort (one shing
visit), the representation of grayling in the overall sh
harvest, sh body sizes (medium body weight), the
percentage of shing grounds with and without harvested
grayling, catch per stocked sh per hectare, and yield per
stocked biomass per hectare. To estimate the effect of sh
stocking on sh catch, data were used on sh stocking
from 3–5 years before the sh were caught. The mean
value of three consecutive years was used in the analysis.
Data on sh stocking from 0–2 years before the catch
were not included because stocked sh were small (10 cm
TL) and unlikely to grow to legal angling size (30 cm)
over two years. Data on sh stocking that were six years
or older were also not included, mainly for two reasons:
(1) the usual lifespan of grayling is 5–6 years maximum,
and (2) stocked sh usually display high mortality due to
stocking stress, predation, angling, and inability to adapt to
natural conditions. Stocked graylings were very unlikely
to survive for six years in the study area. Therefore, the
effect of stocking on catch and yield was calculated based
on data collected within 1991–2015.
Statistical analyses. The statistical programme R (R
i386 3.4.1.; R Development Core Team 2017) was used
for statistical testing. The package for generalized linear
mixed models (GLMM) was used to t the models
(Hadeld 2010). The function lmer in the package lme4
(version 0.99937542; Bates et al. 2015) was used to
calculate R-squared values (Nakagawa et al. 2013). In the
models, catch (sh number), yield (biomass), and body
weight of sh were used as explained variables. The
year, the intensity of sh stocking, and shing visits were
used as explanatory xed variables. The shing ground
variable was used as a random factor. One shing ground
was used as one sample in the analysis. Gamma error
distribution with log link function was used in the models
that described changes over time. The basic equation for
models was
Catch ~ shery + year
In other models, the catch was replaced with yield or
size, and (1|shery) was removed from the analysis in
the case when the model described the number of shing
grounds with and without catches of grayling. All shing
grounds were used in the analysis of sh harvest. Only
shing grounds with non-zero catches of sh (any species)
were used in the analysis of the representation of grayling
in overall catch and yield. Only shing grounds with non-
zero catches of grayling were used in the analysis of size
(body weight) of caught grayling. Only shing grounds
with non-zero stocked grayling were used in the analysis
of the effect of sh stocking on catch and yield. The
minimum probability level of P = 0.05 was accepted for
all the statistical tests, and all statistical tests were two-
tailed. Bonferroni correction was applied when multiple
groups were compared in statistical analysis. The results
presented in the Table 1 are derived from models in R
while the gures were drawn in MS Excel. The method
described above was previously used to analyse similar
data sets on sh harvest (Humpl et al. 2009, Jankovský
et al. 2011, Boukal et al. 2012, Lyach and Čech 2017a,
2018a, 2018b, Lyach and Remr 2019a, 2019b).
RESULTS
Overall summary. Within 1986–2015 (30 years), anglers
caught altogether 9 928 graylings of the total weight of
3 357 kg. In comparison, over 30 years, anglers caught
altogether 7 715 156 sh (of different species) of the
total weight of 11 512.87 t. Within 2005–2015 (11 years),
anglers visited selected shing grounds 5 739 535 times
and caught 1 320 graylings of the total weight of 436 kg.
In comparison, over 11 years, anglers caught altogether
2 234 110 sh (all species) of the total weight of 4 385
t. Anglers visited one hectare of studied shing grounds
238 times, on average, and harvested 0.0096 graylings of
the biomass of 0.0032 kg per hectare of shing grounds
annually. Fisheries managers stocked 1.44 graylings of
the biomass of 0.03 kg per hectare of shing grounds
annually. The results of all used statistical models are
listed in Table 1.
Catch and yield of grayling. Both catch and yield of
grayling decreased to 10% of the initial values over the
course of 30 years (Figs. 1A, 1B). Anglers were catching
0.6 sh and 0.2 kg of sh per hectare of shing grounds
in 1986. However, catch and yield decreased to only 0.07
sh and 0.02 kg of sh per hectare of shing grounds in
the year 2015. The model explained 16% and 5% of the
variability in catch and yield, respectively.
Catch and yield per shing visit. Anglers were catching
fewer grayling per shing effort (shing visit) every year.
Both catch and yield per shing visit decreased to 20%–
25% of the initial values over 11 years (Figs. 2A, 2B).
Anglers caught 0.0003 sh and 0.0001 kg of sh per visit
in 2015. After 11 years, both catch and yield per visit
dropped to 0.00007 sh and 0.00003 kg of sh per shing
visit, respectively, in 2015. The model explained 11% and
12% of the variability in catch and yield, respectively.
Both catch and yield were positively correlated to the
shing effort (intensity of shing visits). Fishing grounds
Lyach and Remr
56
with higher visit rates also displayed higher catch and yield
of grayling. However, shing grounds with the highest
visit rate (35 000–100 000 visits per year) displayed zero
catches of grayling (Figs. 2C, 2D). The positive effect of
shing effort on catch and yield was mostly observed on
shing grounds with lower visit rates (10–10 000 visits
per year). For larger shing grounds, there was a negative
relation between the number of angler shing visits with
both catch and yield of grayling.
Representation in overall catch and yield. The
representation of grayling in the overall catch and yield
of all sh decreased to 20% and 10% of the initial value
(in catch and yield, respectively) over the course of 30
years. The representation of grayling decreased from
0.24% to 0.05% and from 0.08% to 0.008% in catch and
yield, respectively (Figs. 3A, 3B). The model explained
5% of the variability in the representation in both catch
and yield.
Harvest in relation to sh stocking. There was only a
weak correlation between sh stocking and sh harvest.
Higher intensity of sh stocking did not lead to signicantly
higher rates in the sh harvest. Fisheries managers
stocked 5–1000 sh with a total weight of 0.1–27 kg per
one hectare of shing grounds, however, several shing
grounds with high intensity of sh stocking displayed zero
harvested grayling. Inversely, several shing grounds with
low intensity of sh stocking displayed relatively high
harvest rates (considering that overall harvest of grayling
was very low in general).
Fishing grounds with catches. Anglers were catching
grayling on a lower number of shing grounds every
year. The percentage of shing grounds with one or more
harvested grayling was decreasing over time. The number
of shing grounds with catches of grayling decreased to
30% of the initial value (from 12.5% to 3.7%) over 30 years
(Fig. 4A). The model explained 18% of the variability in
the percentage of shing grounds with sh catches.
Size of caught sh. Anglers were catching grayling of
comparable size (body weight) every year. The size of
Table 1
Changes of basic metrics in recreational shing over time with different sheries management (data are for catches
of grayling Thymallus thymallus by recreational anglers in the Czech Republic within 1986–2015)
Dependent variable Explanatory variable Intercept ± SD Slope ± SD P-value Var (RE) R2DF
Catch × ha–1 year 0.35 ± 0.14 –0.048 ± 0.0030 <0.001 2.8200 0.1600 2 681
Yield × ha–1 year 0.19 ± 0.11 –0.016 ± 0.0020 <0.001 1.5940 0.0500 2 681
Catch × shing visit–1 year 4.22 ± 1.14 –0.002 ± 0.0009 <0.001 0.0004 0.1100 2 681
Yield × shing visit–1 year 2.02 ± 0.75 –0.001 ± 0.0003 0.007 0.0002 0.1200 2 681
Catch × ha–1 shing visit 0.023 ± 0.071 0.00004 ± 0.000010 0.002 0.0048 0.04 2 681
Yield × ha–1 shing visit 0.012 ± 0.021 0.00020 ± 0.000004 0.002 0.0039 0.04 2 681
% in overall catch year 27.24 ± 4.37 –0.013 ± 0.0020 <0.001 0.0050 0.0500 2 681
% in overall yield year 24.87 ± 4.49 –0.012 ± 0.0022 <0.001 0.0180 0.0500 2 681
Catch × visit–1 stocked sh n × ha–1 0.00016 ± 0.00007 0.00008 ± 0.00003 0.26 0.00003 0.008 566
Yield × visit–1 stocked b. × ha–1 0.00009 ± 0.00003 0.00003 ± 0.00001 0.21 0.00001 0.003 566
N of sites with catches year 6.78 ± 2.78 –0.003 ± 0.0010 0.016 NA 0.1800 2 681
Mean body weight year –23.82 ± 2.98 0.012 ± 0.0014 0.410 0.0030 0.1800 164
SD = standard deviation, var (RE) = variance for random effect, DF = degrees of freedom, NA = not applicable; N = number; stocked b. =
stocked biomass, stocked sh n = stocked sh number.
Fig. 1. (A) Catch (sh number) and (B) yield (biomass)
of grayling, Thymallus thymallus, per hectare of shing
grounds in the Czech Republic within 1986–2015; the
whiskers represent the standard error of mean
Harvest of grayling versus sheries management 57
caught grayling did not signicantly change over 30 years
(Fig. 4B). The mean size of caught grayling per shing
ground was 0.35 kg and ranged from 0.25 kg to 1.8 kg.
The model explained 18% of the variability in sh size.
DISCUSSION
Data limitations. The dataset that is derived from
individual angling logbooks provided long-term data
on sh catches on a large number of shing grounds,
however, the data should be used and interpreted with
caution. Fish catches are reported by regular anglers
and not by scientists. Since the data are based on citizen
science, the error in the data is probably a bit higher
when compared to real scientic data. On the other hand,
recreational shing connects regular people to nature,
and, to a certain point, to scientic work. That is a big
advantage in a similar type of research. However, this
dataset has several limitations. Anglers may overestimate
or underestimate the numbers and sizes of caught sh,
disobey shing rules, and incorrectly identify harvested
species. Listed errors are made either unknowingly or on
purpose (Essig and Holliday 1991, Pollock et al. 1994,
Cooke et al. 2000, Bray and Schramm 2001, Mosindy
and Duffy 2007, Lyach and Čech 2017a, 2018a, 2018b).
Fisheries data also do not cover poaching or the catch-
and-release shing strategy. Especially salmonids display
high post-release mortality (Clark 1991, Casselman 2005).
However, the dataset provided data on 30 years of catches
on 240 shing grounds, and the data were collected by
approximately 60 000–80 000 different people during
at least tens of millions of working hours (Lyach and
Čech 2018a, 2018b). If the data were collected by a few
scientists, the bias in the selective collection of the data
would be higher, mostly because every person performs
shing a bit differently. It would also be impossible to
collect data on this strength. This dataset was collected
by approximately 60 000–80 000 people in the eld, and
therefore the bias in data collection should be low.
Fig. 2. (A, B) Catch (sh number) and yield (biomass) of grayling, Thymallus thymallus, per shing visit; (C, D) the
relation between shing visit rates and harvest (catch and yield) in the Czech Republic within 1986–2015; the
whiskers (A and B) represent the standard error of mean
Lyach and Remr
58
Political changes. Both catch and yield displayed a
signicant and visible change over the years 1989 and
1990. In 1989, the velvet revolution (fall of the communist
regime) took place in Czechoslovakia. The majority of
metrics in recreational shing were increasing until 1989,
and after that, those metrics started to decrease. Fishing
used to be a very popular leisure activity during the
communist era, mostly because regular people were not
allowed to travel to the western capitalist world (Europe,
North America), and the possibilities of travelling to
eastern Europe were very limited as well. Other means
of entertainment were also signicantly limited. People
shed to obtain food, mostly because food supplies were
also limited and often not available. After 1989, the
borders opened and people could travel, participate in a
wide variety of leisure activities, and buy the food that
they wanted. For that reason, the popularity of shing
decreased, and the number of shing visits also decreased.
That could have caused a decrease in catch and yield. The
agricultural management also changed after 1989; the
input of fertilizers into the environment decreased. That
caused a decrease in primary production in most water
ecosystems (Kunzová and Hejcman 2009). Unfortunately,
data on sh harvest before 1986 were not available.
It seems that both catch and yield increased from 1986
through 1989, and it would be interesting to see when
exactly the increasing trend started. In conclusion, it seems
that the fall of the regime was one of the most important
factors in recreational shing.
Catch and yield. Both catch (sh number) and yield
(biomass) are usually linked to the following three
parameters: population changes of sh species in the
environment, popularity of the catch-and-release shing
strategy, and interest in conservation of sh species
(Jayasynghe et al. 2006, Mosindy and Duffy 2007,
Skov et al. 2017). Those three parameters are also
interconnected; anglers are more likely to release rare
and endangered sh species (Arlinghaus et al. 2007,
Fig. 3. (A, B) The percentage representation of grayling, Thymallus thymallus, in the overall catch (sh number) and yield
(biomass) of all sh caught by anglers in the Czech Republic within 1986–2015; (C) the relation between the amount
of stocked sh per hectare of shing grounds and catch per shing visit; (D) the relation between stocked biomass
per hectare of shing grounds and yield per shing visit; the whiskers (A and B) represent the standard error of mean
Harvest of grayling versus sheries management 59
Bartholomew and Bohnsack 2005). Since anglers are
well aware of the poor population status of grayling, it
is possible that decreased harvest was partially caused
by the increasing popularity of catch-and-release shing.
By studying sheries discussion forums on sheries Web
pages, we found that anglers are strongly supporting the
conservation of grayling. Anglers claim that they are
releasing all caught grayling (Authors’ observation).
When the results of this study are combined with
opinions of local anglers and sheries managers, it can
be concluded that this dataset provided good proxy data
on changes in grayling populations in the study area.
Fishing grounds with catches. The number of shing
grounds with reported catches of grayling were relatively
low already 30 years ago, and the number was decreasing
over time. Grayling is a typical inhabitant of streams, and
the majority of streams in the area are not listed as shing
grounds (Czech Fishing Union, unpublished data). Instead,
they are listed as waters that are used for spawning and
breeding purposes (shing is not allowed there). Streams
in the study area are signicantly affected by pollution,
predation from piscivorous predators (otter Lutra lutra,
cormorant Phalacrocorax carbo, heron Ardea cinerea,
mink Mustela vison), shing pressure, and migration
barriers (Adámek and Jurajda 2001, Humpl and Pivnička
2006, Slavík et al. 2012, Závorka et al. 2013, Lyach
and Čech 2017a, Lyach et al. 2018). Another problem
is a shortage of grayling for stocking purposes. There is
usually not enough grayling to spawn, and therefore the
amount of YOY sh and yearlings available for stocking
is very limited (Czech Fishing Union, unpublished data).
Articial rearing of grayling in aquaculture is signicantly
less protable than the rearing of common carp, Cyprinus
carpio Linnaeus, 1758, or rainbow trout, Oncorhynchus
mykiss (Walbaum, 1792) (see Carlstein 1995). Import of
grayling from abroad is not recommended due to genetic
differences in sh populations (Gum et al. 2009).
Fish stocking. The effect of sh stocking on catch and
yield could be different in areas with pristine unpolluted
streams that support native grayling populations.
Especially streams that are situated in the mountains will
likely show higher catches of grayling. Return rates of
grayling in areas with pristine streams could exceed 100%,
mostly because anglers can catch both native and stocked
grayling there. For example, the biomass of harvested
graylings could be higher when compared to the biomass
of stocked graylings. This effect was observed for self-
reproducing sh populations of very abundant sh species
that are harvested by anglers. For example, European
chub, Squalius cephalus (Linnaeus, 1758), displayed
harvested biomass of 50–100 kg in the Berounka River
(Central Bohemia) even though no stocking of chubs
occurred (Czech Fishing Union, unpublished data).
Similarly, European catsh, Silurus glanis Linnaeus, 1758
displayed harvest rates of 8–10 kg per hectare on the same
river even though no stocking of catsh occurred either
(Lyach and Remr 2019c). There are streams with self-
reproducing grayling populations located under mountains
approximately 100 km from the study area (Horká et al.
2015). However, streams with natural grayling spawning
are rare, and this study describes the situation on typical
lowland streams.
Catch and visit rates. Catch per visit was decreasing even
more rapidly than catch per effort. It is mostly because
anglers were visiting shing grounds more frequently
each year, contributing to increased shing pressure
in the area. As determined by Lyach and Čech (2018a),
the shing pressure has been increasing recently. On the
other hand, both catch and yield have been decreasing
(Lyach and Čech 2018a). Another study found that shing
pressure was the highest on smaller streams where most
grayling are caught (Lyach and Čech 2018b). Therefore,
the effect of recreational shing on grayling populations
could be potentially even more important in the future. The
presently reported study also found that the representation
of grayling in the overall catch was decreasing. Since
Lyach and Čech (2018a) found that the overall sh harvest
Fig. 4. (A) The percentage of shing grounds with and
without a catch of grayling, Thymallus thymallus;
(B) the mean body weight of grayling caught by anglers
in the Czech Republic within 1986–2015; the whiskers
(B) represent the standard error of mean
Lyach and Remr
60
in the study area was, in general, decreasing, the presently
reported study suggests that harvest of grayling has been
decreasing more rapidly when compared to the majority of
other sh species.
Size of caught sh. The size of caught grayling was
constant over time, most likely because anglers are usually
catching sh that are slightly larger than legal angling
size (30 cm TL, total length). According to the length–
weight equations that anglers use to estimate weights of
caught sh, the mean weight of caught grayling (0.35 kg)
represents a 35 cm (TL) specimen. A 30 cm (TL) large
grayling should weigh 0.25 kg.
CONCLUSIONS AND FUTURE PERSPECTIVES
In conclusion, the dataset clearly shows that catch and
yield of grayling have been decreasing over the last 30
years. The decrease in catch and yield can be most likely
explained by population decrease and the increasing
popularity of catch-and-release strategy. Intensive sh
stocking had no signicant effect on harvest rates of
grayling, suggesting that intensive stocking of graylings
was ineffective. Larger shing grounds displayed low
harvest rates of grayling, suggesting that anglers who
want to harvest graylings should focus on smaller-sized
rivers and streams. The data also suggest that the fall of the
communist regime had a signicant effect on recreational
shing, mostly because the harvest of grayling started
decreasing immediately after the changes in the regime
in 1989. This was likely due to new possibilities to travel
abroad and also a higher supply of other recreational
activities. This study provided yet another proof that
conservation of grayling as a species is necessary, mostly
because grayling is slowly vanishing from streams and
rivers in central Europe. We believe that anglers, sheries
managers, and environmentalists should join forces
with the scientic community to nd a way to conserve
grayling populations. We also conclude that angling
logbooks provided a very useful set of data that can be
used in sheries research. We suggest that future studies
should focus on monitoring of streams that still support
self-reproducing grayling populations. Similar studies
would hopefully help to conserve grayling populations for
future generations.
ACKNOWLEDGEMENTS
The Czech Fishing Union (Český rybářský svaz)
(namely Jaroslava Fryšová, Pavel Horáček, and Dušan
Hýbner) provided necessary sheries data. Pavel Vrána,
Martin Čech, Karel Anders, and Robert Arlinghaus
provided helpful insights into recreational shing. Otakar
Ďurďa and Marek Omelka helped with the statistical
analyses. Anglers and angling guards in the Czech
Republic collected data for this study and therefore made
this study possible. This study was nancially supported by
the Charles University Grant Agency (Grant GA UK No.
112 218), by Charles University (Faculty of Science), and
by the Ministry of Education, Youth, and Sports of the
Czech Republic.
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Received: 31 January 2019
Accepted: 11 November 2019
Published electronically: 1 March 2020
... Bonferroni correction was applied in all three models because multiple groups were tested for differences. This method of fisheries data analysis was previously used to analyze fish harvest rates in different research papers [32][33][34][35]. ...
... The angling effort strongly affected the harvest rates of the rheophilic fishes. Previous studies also agreed that the harvest rates of fish are driven by the angling effort [33,35,45,46]. The angling effort is relatively hard to estimate because each angler fishes at a different intensity. ...
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The large area over which European grayling, Thymallus thymallus, extends theoretically prevents it from extinction. However, morphometric and genetic studies have proved that this area is in fact made up of a patch of isolated populations, many of which are highly threatened or have already been eradicated by river alterations. Stocking often appears inefficient because of the low quality of the watercourses, and may also have important genetic implications. A loss of genetic specificity of local wild strains may induce a reduction in their fitness. In France several populations seem to be contaminated by allochthonous genes, but, in good environments, wild strains seem to better resist the introgression. Production of offspring in small hatcheries from wild spawners collected in the river to be stocked is promoted to conserve the genetic identity of local strains. Nonetheless, definite protection will not be expected without the rigorous protection of the habitat for each population.